To distinguish between learned and unlearned responses is not always an easy task. It cannot be assumed that responses which are absent during the first weeks of life are all learned. To take the most obvious instance; sexual responses change their character to a greater or less extent at puberty, as a result of changes in the ductless glands, not as a result of experience. But this instance does not stand alone: as the body grows and develops, new modes of response come into play, modified, no doubt, by experience, but not wholly due to it. For example: a new-born baby cannot run, and therefore does not run away from what is terrifying, as an older child does. The older child has learned to run, but has not necessarily learned to run _away_; the stimulus in learning to run may have never been a terrifying object. It would therefore be a fallacy to suppose that we can distinguish between learned and unlearned responses by observing what a new-born infant does, since reflexes may come into play at a later stage. Conversely, some things which a child does at birth may have been learned, when they are such as it could have done in the womb--for example, a certain amount of kicking and stretching. The whole distinction between learned and unlearned responses, therefore, is not so definite as we could wish. At the two extremes we get clear cases, such as sneezing on the one hand and speaking on the other; but there are intermediate forms of behaviour which are more difficult to classify. This is not denied even by those who attach most importance to the distinction between learned and unlearned responses. In Dr. Watson’s _Behaviorism_ (p. 103) there is a “Summary of Unlearned Equipment”, which ends with the following paragraph: “Other activities appear at a later stage--such as blinking, reaching, handling, handedness, crawling, standing, sitting-up, walking, running, jumping. _In the great majority of these later activities it is difficult to say how much of the act as a whole is due to training or conditioning. A considerable part is unquestionably due to the growth changes in structure, and the remainder is due to training and conditioning._” (Watson’s italics.)